Cvd and pvd

1. ECE614: Device Modelling
and Circuit Simulation
Unit 2 PVD & CVD
By Dr. Ghanshyam Singh

2. Content

Physical vapor deposition
(PVD)
– Thermal evaporation
– Sputtering
– Evaporation and sputtering
compared
– MBE
– Laser sputtering
– Ion Plating
– Cluster-Beam

Chemical vapor deposition
(CVD)
– Reaction mechanisms
– Step coverage
– CVD overview

Epitaxy

3. Physical vapor deposition (PVD)

The physical vapor deposition technique is based on the formation of vapor of
the material to be deposited as a thin film. The ma terial in solid form is either
heated until evaporation (thermal evaporation) or s puttered by ions
(sputtering). In the last case, ions are generated by a plasma discharge usually
within an inert gas (argon). It is also possible to bombard the sample with an
ion beam from an external ion source. This allows t o vary the energy and
intensity of ions reaching the target surface.

4. Physical vapor deposition (PVD):
thermal evaporation
Heat Sources Advantages Disadvantages Resistance No radiation Contamination e-beam Low contamination Radiation RF No radiation Contamination Laser No radiation, low
contamination
Expensive
N =
N
o
exp-
Φ
e
kT
6
The number of molecules
leaving a unit area of evaporant
per second
N
o = slowly varying function of T
ϕ
e= activation energy required to
evaporate one molecule of
material
k = Boltzman’s constant
T = temperature

5. Physical vapor deposition (PVD): thermal
evaporation
Si
Resist
d
ββββ
θθθθ
Evaporant container
with orifice diameter D D
Arbitrary
surface element
K
n
=
λλλλ
/D > 1
A ~ cosββββ cos θθθθ/d
2
N

(
m
o
l
e
c
u
l
e
s
/
u
n
i
t

a
r
e
a
/
u
n
i
t

t
i
m
e
)

=
3.513.10
22
P
v(T)/ (MT)
1/2
The cosine law
This is the relation between vapor pressure of
the evaporant and the evaporation rate. If a high
vacuum is established, most molecules/atoms will re ach
the substrate without intervening collisions. Atoms and
molecules flow through the orifice in a single stra ight
track,or we have free molecular flow :
The fraction of particles scattered by collisions
with atoms of residual gas is proportional to:
The source-to-wafer distance must be smaller than t he mean free path (e.g, 25 to 70 cm)

6. Physical vapor deposition (PVD): thermal
evaporation
ββββ2222 = 70  = 70  = 70  = 70
0000
ββββ1111 = 0  = 0 = 0  = 0
0000
t2
t1
Substrate
t
1
t
2
=
cos
ββββ
1
cos ββββ
2
≈≈≈≈ 3
Surface feature
Source
Source
Shadow
t
1
/
t
2
=
c
o
s
ββββ
1111
/
c
o
s
ββββ
2222
λλλλ = (ππππRT/2M)
1/2
ηηηη/P
T
From kinetic theory the mean free path relates
to the total pressure as:
Since the thickness of the deposited film, t, is pr oportional
to the cos β, the ratio of the film thickness shown in the
figure on the right with θ= 0°is given as:

7. Physical vapor deposition (PVD): sputtering
W=
kV i P
T
d
-V working voltage
- i discharge current
- d, anode-cathode distance
- P
T, gas pressure
- k proportionality constant
Momentum transfer

8. Evaporation
and
sputtering:
comparison
Evaporation Sputtering
RateThousand atomic layers per second
(e.g. 0.5 µm/min for Al)
One atomic layer per second Choice of materialsLimited Almost unlimited PurityBetter (no gas inclusions, very high
vacuum)
Possibility of incorporating
impurities (low-medium vacuum
range)
Substrate heatingVery low Unless magnetron is used substrate
heating can be substantial
Surface damageVery low, with e-beam x-ray
damage is possible
Ionic bombardment damage In-situ cleaningNot an option Easily done with a sputter etch Alloy compositions,
stochiometry
Little or no control Alloy composition can be tightly
controlled
X-ray damageOnly with e-beam evaporation Radiation and particle damage is
possible
Changes in source
material
Easy Expensive Decomposition of
material
High Low Scaling-upDifficult Good UniformityDifficult Easy over large areas Capital EquipmentLow cost More expensive Number of
depositions
Only one deposition per charge Many depositions can be carried
out per target
Thickness controlNot easy to control Several controls possible AdhesionOften poor Excellent Shadowing effectLarge Small Film properties (e.g.
grain size and step
coverage)
Difficult to control Control by bias, pressure,
substrate heat

9. Physical vapor deposition (PVD): MBE,
Laser Ablation
-

MBE
– Epitaxy: homo-epitaxy
hetero-epitaxy
– Very slow: 1µm/hr
– Very low pressure: 10
-11
Torr

Laser sputter deposition
– Complex compounds (e.g.
HTSC, biocompatible
ceramics)

10. Chemical vapor deposition (CVD): reaction
mechanisms

Mass transport of the reactant in
the bulk
Gas-phase reactions
(
homogeneous
)

Mass transport to the surface

Adsorption on the surface

Surface reactions
(
heterogeneous
)

Surface migration

Incorporation of film
constituents, island formation
Desorption of by-products

Mass transport of by-produccts
in bulk
CVD: Diffusive-convective transport of
depositing species to a substrate
with many intermolecular
collisions-driven by a concentration
gradient
SiH4
SiH
4
Si

11. Chemical vapor deposition (CVD):
reaction mechanisms
Fl = D
∆
c
∆
x
δ(x)=
ηx
ρU
 
 
 
 
 
 
12
δ =
1
L
δ(x)dX=
2
3
0
L
∫
L
η
ρUL
 
 
 
 
 
 
12
Re
L
=
ρ
UL η
δ
=
2L
3
R
e
L

Energy sources for deposition:
– Thermal
– Plasma
– Laser
– Photons

Deposition rate or film growth rate
(Fick’s first law)
(gas viscosity η, gas densityρ, gas stream velocity U)
(Dimensionless Reynolds number)
Laminar flow
L
δ(x)
dx
(U)
(Boundary layer thickness)
Fl = D
∆
c
2
L
3 Re
L
(by substitution in Fick’s first law and ∆x=δ)

12.
Mass flow controlled regime
(square root of gas
velocity)(e.g. AP CVD~ 100-10
kPa) : FASTER

Thermally activated regime:
rate limiting step is surface
reaction (e.g. LP CVD ~ 100
Pa----D is very large) :
SLOWER
Chemical vapor deposition (CVD)
: reaction mechanisms
Fl = D
∆
c
2
L
3 Re
L
R = R
o
e
-
E
a
kT

13. Chemical vapor deposition (CVD):
step coverage
Fl = D
∆
c
2
L
3 Re
L
R = R
o
e
-
E
a
kT

Step coverage, two factors are
important
– Mean free path and surface
migration i.e. P and T
– Mean free path: λ =
αααα
w
z
θ=180 θ=180 θ=180 θ=180
0000
θ=270 θ=270 θ=270 θ=270
0000
θ=90 θ=90θ=90 θ=90
0000
θ is angle of arrival
kT
2
1
2
P
Tπa
2
> α
Fld
θ
∫
θ
=arctan
wz

14. Chemical vapor deposition (CVD) :
overview

CVD (thermal)
– APCVD (atmospheric)
– LPCVD (<10 Pa)
– VLPCVD (<1.3 Pa)

PE CVD (plasma enhanced)

Photon-assisted CVD

Laser-assisted CVD

MOCVD

15.
The L-CVD method is able to fabricate
continuous thin rods by pulling the substrate
away from the stationary laser focus at the
linear growth speed of the material while
keeping the laser focus on the rod tip, as shown
in the Figure . LCVD was first demonstrated
for carbon and silicon rods. However, fibers
were grown from other substrates including
silicon, carbon, boron, oxides, nitrides,
carbides, borides, and metals such as
aluminium. The L-CVD process can operate at
low and high chamber pressures. The growth
rate is normally less than 100 µm/s at low
chamber pressure (<<1 bar). At high chamber
pressure (>1 bar), high growth rate (>1.1
mm/s) has been achieved for small-diameter (<
20 µm) amorphous boron.
Chemical vapor deposition (CVD) : L-CVD

16. Epitaxy

VPE:
– MBE (PVD) (see above)
– MOCVD (CVD) i.e.organo-metallic
CVD(e.g. trimethyl aluminum to
deposit Al) (see above)

Liquid phase epitaxy

Solid epitaxy: recrystallization of
amorphous material (e.g. poly-Si)
Liquid phase epitaxy

17. Epitaxy

Selective epitaxy

Epi-layer thickness:
– IR
– Capacitance,Voltage
– Profilometry
– Tapered groove
– Angle-lap and stain
– Weighing
Selective epitaxy

18. Homework

Homework: demonstrate equality of λ= (πRT/2M)
1/2
η/P
T and λ= kT/2
1/2
a
2
πP
T
(where a is the molecular diameter)

What is the mean free path (MFP)? How can you incre ase the MFP in a vacuum
chamber? For metal deposition in an evaporation sys tem, compare the distance
between target and evaporation source with working MFP. Which one has the
smaller dimension? 1 atmosphere pressure = ____ mm Hg =___ torr. What are the
physical dimensions of impingement rate?

Why is sputter deposition so much slower than evapo ration deposition? Make a
detailed comparison of the two deposition methods.

Develop the principal equation for the material flu x to a substrate in a CVD process,
and indicate how one moves from a mass transport li mited to reaction-rate limited
regime. Explain why in one case wafers can be stack ed close and vertically while in
the other a horizontal stacking is preferred.

Describe step coverage with CVD processes. Explain how gas pressure and surface
temperature may influence these different profiles.